SEATTLE  Mount St. Helens spewed more steam and ash Monday, as government scientists remained on alert for a larger eruption at the Washington state volcano, which woke last week after 18 years of slumber.

Mount St. Helens, which in 1980 killed 57 people when it erupted violently, continued to increase its activity, following a week of tremors and an minor eruption Friday.

"We could go into a more substantial event without warning," Willie Scott, a U.S. Geological Survey geologist, told reporters.

The 1980 eruption of Mount St. Helens was one of the most devastating in modern history. In addition to the heavy casualty toll, it destroyed more than 200 homes and flattened acres of evergreen spruce forest. The volcano continued to have a series of smaller eruptions until 1986.

The U.S. Geological Survey kept its warning level at a Level 3-Volcano Alert and kept off-limits a visitor center at the Johnston Ridge Observatory about five miles from the volcano's crater as a safety precaution.

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Government scientists said that low-frequency earthquakes, carbon dioxide gas, and swelling in the lava dome created after the 1980 eruption all pointed toward the buildup of magma under the mountain.

Although scientists have said that they do not expect an explosion that would cause any deaths, they are concerned about the impact of any ash on air traffic and humans following an explosion.

Mount St. Helens is located about 100 miles south of Seattle and 50 miles north of a busy airport at Portland, Oregon.

A thick plume of steam rose from the crater of Mount St. Helens Monday at around 9:30 a.m., following a similar discharge 11 hours earlier.

Scott also said that the lava dome in the volcano's crater, as well as a glacier nestled next to it, had risen significantly.

The Federal Aviation Administration also notified nearby aircraft of Monday's steam eruption and directed them to avoid the steam and ash plume, said Mike Fergus, an agency spokesman. Aircraft engines can be stalled by ash.

The violent blast in 1980 blew off the top of the mountain and reduced the summit of Mount St. Helens to 8,364 feet from 9,677 feet.

Smaller eruptions in the lava dome happened in 1986 but caused no serious damage.